1. Field of the Invention
This invention relates to machines for making plastic bags or the like from a continuous web of material and, more specifically, to machines comprising a radially adjustable sealing drum and means for adjusting the sealing drum to maintain a desired spacing between the seals imparted to the web by the sealing drum and any registration mark appearing on the web. More particularly, the invention relates to a machine which further comprises means for perforating the web to enable individual bags to be subsequently separated from the film and means for automatically maintaining a desired spacing between the seals and the perforations.
2. Description Of Related Art
In existing bag making machines, a continuous film is drawn from a source, such as a roll of plastic tubing, and is fed into a sealing drum and blanket assembly where transverse seals are impaled to the film to define individual plastic bags. The film then travels through various optional stations, such as a handle punching station and a folding board assembly, where further operations are performed on the film. Finally, the film is conveyed through a perforator, which perforates the film transversely of the direction of travel so that the individual bags can be subsequently separated from the film. The perforations are placed adjacent the seals and, to avoid wasting material, the distance between the perforation and the seal, which is referred to as the "skirt", should be kept at a desired minimum. Also, in twin seal bags, which are open transverse to the direction of travel and have a seal defining each side, the perforation is located between the seals defining adjacent sides of consecutive bags. In order to avoid wasting material in the production of this type of bag, the adjacent seals should be located a minimum distance apart and, therefore, care must be taken to consistently locate the perforation between the seals.
In many applications it is desired that printed matter appear on the individual bags. In these instances, the source of the film may comprise a continuous roll of tubing having preprinted matter imparted thereon at spaced intervals corresponding to the desired size of the bags. Furthermore, it is typically required that the printed matter appear at the same location on the individual bags from bag to bag. This requirement is usually addressed by maintaining a fixed distance from the printed matter to the seal on each bag. However, since the locations of the preprinted matter on the tubing may vary due to certain factors in the production and printing of the tubing, it is often difficult to maintain a fixed distance between the seal and the printed matter.
Apparatus for automatically varying the placement of the location of the seals to maintain a fixed distance between the seals and the printed matter is disclosed in U.S. Pat. No. 4,934,993, issued to Gietman, Jr. In Gietman, Jr., the film contacting surface of the sealing drum comprises a number of slats and one or more seal bars. The diameter of the drum is variable in response to a motor located within the drum which is connected through a series of gears and chains to a number of threaded rods supporting the ends of the slats and seal bars. A first detector detects a registration mark appearing on the film at regular intervals in spaced relation to the printed matter and a second detector generates a signal representative of one revolution of the sealing drum. A CPU then compares the relationship between these signals with certain preset conditions and, if necessary, activates the sealing drum motor to vary the diameter of the sealing drum and thereby change the relationship between the seals and the printed matter until a desired constant is arrived at and maintained.
However, in Gietman, Jr. and other prior art bag making machines, the perforator is driven by the sealing drum and the location of the perforation relative to the seal is dependent upon the diameter of the sealing drum. Thus, while the distance between the seal and the perforation can be initially manually set, automatically varying the diameter of the sealing drum to maintain a desired relationship between the seal and the printed matter will consequently alter the distance between the seal and the perforation. In prior art bag making machines, the operator is required to manually adjust the perforator to maintain the proper distance between the seals and the perforations if any changes have occurred. For example, Gietman, Jr. discloses using a hand-operable variator to do this. However, since automatically adjusting the location of the seals in relation to the printed matter can result in repeated changes in the location of the seals, manually adjusting the perforator is not practical. To compensate for not having to continually adjust the perforator, the distance between the perforations and the seals is typically selected to be large enough to accommodate certain variations in the location of the seals. However, given the large volume of bags usually produced in a given production run, these large skirt sizes result in a great deal of material waste.